The invention relates to a process and systems for measuring physical quantities of poorly conductive and nonconductive fluids such as rolling oil, lubricating oil, hydraulic fluid, brake fluids and fuels, the fluid being located between or in the field of action of at least two electrically conductive electrodes.
In the area of the rolling oil system of the primary tank of rolling units and in the area of roll stands deflagration and fires caused by electrostatic discharges occasionally occur.
Formation of electrostatic charges due to friction effects occurs in rolling oil in spite of grounding of the roll stands and all involved components when the electrical conductivity of the oil drops below a minimum value, so that balancing of electrical charges is no longer ensured.
To counteract the danger of electrostatic discharge processes and the associated danger of fire in rolling mills, the electrical conductivity of the rolling oil should be continuously checked. When the value drops below a certain threshold a conductivity additive can optionally be added to the oil.
Measuring devices are commercially available with which the electrical conductivity of fluids can be determined using different processes. Existing conductive measuring instruments for nonconductive or poorly conductive fluids such as oils or fuels are not suited for measurement in flowing media (online operation). The fluid to be measured must be at rest for several minutes in the known systems for the measuring instrument to display a reliable value. Running extraction of measurement samples is time-consuming and labor-intensive and is associated with the corresponding costs. Another disadvantage of offline measuring instruments is also that measured values of electrical conductivity of only limited reliability can be obtained since conductivity can be changed by many influences within a short time. When a value in the danger range is being measured efforts are made to reduce it again by adding additives, for example, to rolling oil, so that frictionless production can be ensured. Since the time intervals between sampling and the measuring instant are considerable due to the rest time of the oil prescribed by the measuring instrument manufacturer, under certain circumstances it can already be too late to eliminate the danger of electrostatic discharges.
This applies especially to a mechanical filtering process in which the abraded rolled material is filtered out of the rolling oil and large amounts of electrical charge are produced by charge separation or frictional electricity due to division and branching of the oil flow via the pores and fine capillaries of a filtering agent such as kieselguhr.
One major disadvantage of commercially available measuring instruments for measuring electrical conductivity of poorly conductive or nonconductive fluids is that the inevitable parasitic capacitances of the sensors used in the measuring instruments, like polarization capacitances, cause major measurement deviations.
To measure the fouling of fluids optical measuring devices are commercially available with which the cloudiness is measured to determine the degree of fouling of the fluids. But in the meantime it has been ascertained that cloudiness is not a measure of the degree of fouling of many fluids, for example, oils, since the geometrical dimensions of the dirt particles contained in the fluid are to some extent smaller than the wavelength of the light, so that these extremely small dirt particles are no longer recorded by measurement of cloudiness based on light refraction.
The faulty manner of operation of commercially available devices for measurement of electrical conductivity and the degree of fouling of oils, fuels and similar nonconductive and poorly conductive fluids leads to agents for increasing the electrical conductivity of fluids such as rolling oil being added to them preventatively and filtering agents such as kieselguhr and bleaching earth being replaced long before or shortly after their exhaustion when oils are being filtered. Known agents which are also called conductivity additives for increasing conductivity are suspect both ecologically and in terms of health so that wholesale preventative addition of these agents causes high environmental burdens and disposal problems. High filtrate quality caused by increased use of filtering agents and reactions of the conductivity additive with filtering aids can greatly reduce the conductivity of oil and oil products, and due to reduced conductivity high electrical charges occur which can lead to uncontrolled deflagration and burning of machinery and plants.